Formerly, one communication device was able to communicate with another only in one frequency band. In recent years, however, a communication device that can communicate in a plurality of frequency bands has been developed. For example, in a wireless LAN, a communicating system using a 2.4-GHz band and a communicating system using a 5-GHz band are available. Also in a mobile telephone, a system using a 0.8-GHz band and a system using a 1.5-GHz band are available. Such a communication device that can communicate in a plurality of frequency bands uses a multi-band antenna capable of transmitting and receiving radio waves of a plurality of frequency bands.
Various types of multi-band antennas are available. For example, an antenna 300 shown in FIG. 9 has two antenna elements 304 and 306 made of conductors placed in parallel on a dielectric substrate 302. The antenna elements 304 and 306 are connected to two branches into which a feeder line 308 is divided at an intermediate point from a signal source (not shown). (For example, refer to a document, “Zukai idoutuushinyou antenna system” written by FUJIMOTO Kyouhei, YAMADA Yoshihide, and TUNEKAWA Kouichi, published by Sougou Denshi Shuppansha, First edition, Oct. 10, 1996)